[RFC] dma-buf: Import/export the implicit fences on the dma-buf

[Daniel Vetter]

On Tue, Jul 12, 2016 at 01:14:41PM +0100, Chris Wilson wrote:
> On Tue, Jul 12, 2016 at 01:53:56PM +0200, Christian König wrote:
> > Am 11.07.2016 um 23:59 schrieb Chris Wilson:
> > >When dealing with user interfaces that utilize explicit fences, it is
> > >convenient to sometimes create those fences after the fact, i.e. to
> > >query the dma-buf for the current set of implicit fences, encapsulate
> > >those into a sync_file and hand that fd back to userspace.
> > >Correspondingly, being able to add an explicit fence back into the mix
> > >of fences being tracked by the dma-buf allows that userspace fence to be
> > >included in any implicit tracking.
> > 
> > Well I think that this is a rather questionable interface.
> > 
> > For example how do you deal with race conditions? E.g. between
> > querying the fences from the reservation object and adding a new one
> > we could gain new fences because of the kernel swapping things out
> > or another thread making some submission with this buffer.
> > 
> > Additional to that IIRC reservation_object_add_excl_fence()
> > currently replaces all shared fences with the exclusive one. A
> > malicious application could use this to trick the kernel driver into
> > thinking that this buffer object is idle while it is still accessed
> > by some operation. At least with GPU operations you can easily take
> > over the system when you manage to get access to a page table with
> > this.
> 
> The only difference here is that we believe the GPU drivers to enforce
> the ordering between each other. So we can either insert a wait before
> adding the exclusive fence, or we can just not expose an import ioctl.
> Extracting the set of fences isn't an issue? (That's the part that has a
> more legitimate usecase.)

If we change the kernel to just merge everything together when importing a
new fence I think it should be perfectly save. I.e.
1) grab reservation lock
2) assemble a fence_array with the current fences + the new one passed in
through sync_file.
3) put that into the right slot
4) unlock

If we switch reservations over to fence_array it might even be somewhat
pretty.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch